Our objectives are to use radio interferometry to study composition, velocity distribution, maser excitation, and plasma interactions of cometary gas. Two new cometary chemistry programs were started with radio interferometers: (1) the VLA used to search for HC3N emission from Comet Brorsen-Metcalf at 3.3 cm wavelength; and (2) the BIMA millimeter array used to observe Comet Austin in HCN

Depater, Imke

Palmer, Patrick

Snyder, Lewis E.

English

NASA Technical Reports Server

N92-12845
Radar lnterferometric Studies of Comets -"
Department of Astronomy
University of Illinois
231 Astronomy Building
1002 West Green Street
Urbana, IL 61801
Lewis E. Snyder (PI), University of Illinois
Patrick Palmer (Co-I), University of Chicago
lmke De Pater (Sci Coil), University of California, Berkeley
Strategy
Our objectives are to use radio interferometry to study the composition, velocity distribution,
maser excitation, and plasma interactions of cometary gas. In previous studies, we
demonstrated that the VLA can be used to make radio images of 18 cm wavelength cometary
OH emission with a resolution of = 1' which show previously unknown and unexpected
small scale structure close to the nucleus. We used similar techniques with the VLA to detect
the 6 cm transition of H._CO in comets Halley, Machh_lz, and Brorsen-Metcalf. We
discovered that the Comet Halley H_CO emission was prodiJced from an extended source in
the coma as well as directly from the nucleus; this was later confirmed by Krankowsky's
analysis of the H2CO channel in the NMS data. The centimeter wavelength detections place
important constraints on the partition function and excitation of cometary H2CO; these
constraints are essential for interpreting observati_ms at shorter wavelengths and, in particular,
for deriving the correct gas production rate from radiative transfer models.
Two new cometary chemistry pr¢,grams have been started with radio intcrfercm'_etcrs. First,
the VLA has been used to search for HC._N emission from Comet Brorsen-Metcalf at 3.3 cm
wavelength, and it was denmnstrated thai for this molecule the VLA can be expected to reach
significant levels of sensitivity in many comets. Second, the BIMA millimeter array was used
to observe Comet Austin in HCN while our colleagues observed HCN simultaneously with
the FCRAO 14-m radio telescope and the Flagstaff 42-inch optical telescope. The primary
goal was to map the distribution of cometary HCN, compare the radio data with the
simultaneous, high resolution optical CN images, and solve the long-standing problem of
whether HCN is the parent molecule of optical CN. The secondary goal was t¢, test the
feasibility of detecting the 3 mm continuum emissicm from ccm_ets. Both programs have had
a modest amount of success, which demonstrates some imp_rtant directi_ms f_r future radio
interferometric studies of c_mletary chemistry.
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Publications
Snyder, L. E., Palmer, P., and de Pater, I., 1990, "Observations of Formaldehyde in Comet
Machholz (1988j)", Icarus, 86, 289-298.
Palmer, P., A'Hearn, M. F., de Pater, I., Klavetter, J. L., Mehringer, D., Schloerb, F. P.,
Snyder, L. E., and Wilner, D., 1990, "Simultaneous Imaging of Optical CN Lines and
Radio HCN Lines in Comet Austin", in Workshop on Observations of Recent Comets
(1990), eds. W. F. Huebner, J. Rahe, P. A. Wehinger, and I. Konno (Southwest
Research Institute), p. 40-43.
Snyder, L. E., Palmer, P., and de Pater, I., 1990, "VLA Searches for Formaldehyde and
Cyanoacetylene Emission from Comet P/Brorsen-Metcalf (1989o)", in Workshop on
Observations of Recent Comets (1990), eds. W. F. Huebner, J. Rahe, P. A. Wehinger,
and I. Konno (Southwest Research Instilute), p. 86-91.
de Pater, I., Palmer, P., and Snyder, L. E., 1990, "A Review of Radio Interferometric Imaging
of Comets", in Comets in the P,st-Hallev Era, eds. R. L. Newburn, M. Neugebauer,
and J. Rahe (Dordrecht:Kluwer), in press.
Snyder, L. E., Palmer, P., and de Pater, I., 1990, "Comet Halley and Interstellar Chemistry",
in Proceedin_os of Pacifichem '89 Symposium No. 228, Chemistry and Spectroscopy of
Interstellar Molecules, ed. N. Kaifu (University of Tokyo Press), in press.
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Radar interferometric studies of comets

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